Reference voltage circuits for use between a voltage supply and an input terminal to a reference amplifier or comparator have in the past included temperature compensating components or circuits. Most commonly the voltage supply is connected to a diode tied to ground through a resistor. The diode is oriented with its bias to pass current to ground. With this circuit, the diode side of the supply resistor becomes the voltage output terminal which is connected to an input terminal of the reference amplifier or comparator. The .6V voltage drop across the diode provides a relatively stable reference voltage into the amplifier.
The problem with this circuit, however, is that the reference voltage supplied to the input terminal of the amplifier or comparator while being tied down or limited by the diode does in fact drift because of the temperature coefficient of the diode. Norminally, this drift may be as much as 3 millivolts per degree centigrade.
One engineering approach which compensates for the temperature coefficient of the diode is the use of two closely matched diodes connected in series to ground. The purpose is to create a voltage divider with the diodes and to rely upon the diodes drifting in a similar manner with temperature. If this occurs, the ratio will be maintained which ultimately presents the same voltage value to the amplifier. However, such diode pairing becomes quite expensive and is cost prohibitive in many circuits.
An equivalent circuit to the dual diode approach above is a single transistor which has been connected to act as the two closely matched diodes. In this circuit, the collector-base junction forms one diode and the base-emitter junction forms the other. A single transistor has, thusly, been connected with its collector and base terminals tied together and its emitter terminal tied to ground. For this circuit, a voltage supply is tied to the collector through a current limiting supply resistor and the collector is designated the output terminal of the circuit. However, as with the previous circuit, the selection of an appropriate transistor having closely matched collector-base and base-emitter diode junctions with matched temperature coefficients becomes expensive. Moreover, even with the most selectively chosen transistors, temperature drift still occurs in the range of 200 to 300 parts per million or 0.02 to 0.03% change over the operating temperature range.
An object of this invention, therefore, is to provide a temperature reference circuit having a very low overall temperature coefficient, approaching zero temperature coefficient.
A second object of this invention is to provide such a circuit which may be constructed of inexpensive and readily available components.
A further object is to provide this zero temperature coefficient reference circuit using inexpensive components wherein temperature compensation is provided to counteract and nullify the effect of the normal temperature drift in the inexpensive components.